docs: def findlayer(self, z):
          '''
          Returns layer-number, layer-type and model-layer-number'''
          if z > self.z[0]:
              modellayer, ltype = -1, 'above'
              layernumber = None
          elif z < self.z[-1]:
              modellayer, ltype = len(self.layernumber), 'below'
              layernumber = None
          else:
              modellayer = np.argwhere((z <= self.z[:-1]) & (z >= self.z[1:]))[0, 0]
              layernumber = self.layernumber[modellayer]
              ltype = self.ltype[modellayer] 
          return layernumber, ltype, modellayer